1. Field of the Invention
This invention relates to a screw charger for charging wet batch material into a high temperature material processing furnace, in particular, a glass melting furnace.
2. Description of Prior Art
For certain manufacturing applications involving the processing of particulate batch materials, introduction of moisture to the dry batch materials is advantageous and, in some cases, even necessary, in order to prevent high-velocity furnace gases from picking up the particulate and depositing it on the furnace superstructure or within heat exchange media of the regenerator. Such deposits can cause, respectively, deterioration of the superstructure, and, through plugging of the heat exchange media, loss of efficiency in the furnace. In addition, melting of the batch is affected negatively by separation of the batch components. Numerous devices are known for mixing a dry particulate material with a liquid, such as water, and transporting the wet mixture to a desired location. For example, U.S. Pat. No. 897,495 teaches an apparatus for the production of hydrate of lime comprising a cylindrical slaking receptacle, a charging hopper, and a shaft having mixing and transporting blades, or paddles, for conveyance of a pasty hydrate of lime from the hopper end of the cylindrical slaking receptacle through the cylindrical slaking receptacle and out through an outlet opening thereof. The use of paddles mounted at an oblique angle on a shaft disposed within an elongated tub for mixing molding sand is taught by U.S. Pat. No. 3,964,732 while U.S. Pat. No. 4,483,625 teaches a method and apparatus for increasing the capacity of paddle screw mixer conveyors upon creating a dam of mixed particulate matter and another substance at the outlet end of the mixture. In accordance with one embodiment, two counter rotating shafts of paddles are employed to increase mixing efficiency.
U.S. Pat. No. 4,172,712 teaches an apparatus for automatically wetting and mixing glass batch immediately prior to its introduction into a glass melting furnace. The apparatus includes a structural arrangement wherein the typical hopper of a glass furnace charger is enlarged to such an extent that it will accommodate a mixing arrangement. Mixing action within the hopper is achieved by means of two rotating shafts, each of which is equipped with a plurality of paddle members. The apparatus further includes a fluid delivery system for delivering water to the batch material.
It is particularly desirable that, in the case of some glass batches, moisture be added to the batch at a point as close as possible to the point at which the batch is deposited into the glass melting furnace. Addition of the moisture at such a point is critical because certain batch constituents, upon addition of moisture, begin to harden, much as portland cement hardens after the addition of water.
Glass batch in its usual form is a mixture of finely divided solids which are thoroughly mixed and delivered to a refractory furnace by a system of hoppers, gravity flow chutes and other positive displacement conveyors. Because the batch is a finely divided material, severe dusting conditions are commonly encountered when the batch is exposed to high velocity hot gases such as those encountered in glass melting furnaces. In addition, the glass batch is extremely abrasive and will erode even the hardest of materials in a relatively short period of time where it frictionally contacts the moving parts of conventional positive displacement conveyance means such as screw conveyors, augers, and the like.
Numerous attempts have been made to reduce furnace dust conditions by wetting the glass batch prior to its delivery to the glass melting furnace. Such techniques, however, typically create additional problems, particularly as they relate to caking within the various conveyors and other transportation means which commonly form a part of the batch handling system. Process and apparatus improvements intended to alleviate this caking problem have included batch cooling and conveyor mixing. It will also be apparent to those skilled in the art that in those situations where batch temperature control is a requirement, the various controllers and heat transfer equipment will present economic considerations that make such improvements prohibitive. In addition, in those instances where the batch is being actively conveyed by the mixer apparatus, undue equipment wear is encountered due to the abrasive characteristics of the batch.
Accordingly, it is one object of this invention to provide a method and apparatus for effectively wetting glass batch material which avoids or overcomes the problems associated with batch wetting set forth hereinabove.
It is another object of this invention to provide an apparatus for wetting raw batch material and conveying the wetted raw batch material into a material processing furnace.
These and other objects of this invention are addressed by a wet batch material charging apparatus comprising a charger screw housing having a batch material inlet, a batch material outlet or discharge and a dry batch material supply means adapted to convey a dry batch material through the batch material inlet. Disposed within the charger screw housing is a rotatable charger screw shaft comprising at least one screw flight and adapted to convey the batch material from the batch material inlet to the batch material outlet. Wetting means are provided for introducing water or other liquid into the charger screw housing. The wetting means comprises flow control means for independently controlling a liquid flow rate and a liquid flow duration based upon the batch material charging speed and rotation of the rotatable charger screw shaft. In accordance with one embodiment, the wetting means comprises at least one liquid spray nozzle positioned to introduce the liquid into the charger screw housing, which liquid spray nozzle is operably connected to the flow control means, which, in turn, comprises means for enabling intermittent flow of the liquid through the liquid spray nozzle based upon rotation of the rotatable charger screw shaft.
In accordance with the method of this invention for charging wet batch material into a material processing furnace, dry batch material is introduced into the charger screw housing in which the charger screw shaft comprising at least one screw flight is rotating. A pulsing stream of a liquid, such as water, is introduced into the charger screw housing through a liquid spray nozzle immediately after the screw flight due to rotation of the rotatable charger screw shaft has passed by the liquid spray nozzle, that is, proximate the trailing edge of the screw flight. The resulting wet batch is then discharged from the charger screw housing into the material processing furnace.
At the trailing edge of the screw flight, there is a batch void or low density zone. The presence of this zone enables better liquid penetration into the batch material, thereby facilitating batch and liquid contact. This is important for wetting certain batch types, such as borosilicate glass batch, which has a tendency to set-up or harden relatively quickly upon wetting. However, the better penetration of liquid into the batch material resulting from the method of this invention enables placement of the liquid spray nozzles closer to the batch material discharge while still providing thorough mixing of the batch and liquid.